You are not wrong. In 2021, I did bootstrap everything on my own – dataset, segmentation algorithm, neural net, UI, math engine, etc. And in the end, it worked

You are not wrong. In 2021, I did bootstrap everything on my own – dataset, segmentation algorithm, neural net, UI, math engine, etc. And in the end, it worked properly only for me. But that was my aim – own complete stack to be able to try out everything that might be interesting without being burdened by 3rd party solutions. It was a prototype that I could show to people – spread the idea. Doing product of that magnitude on my own would never fly. I don't think Apple did a "cool tech demo". In my view, they published first version of a very ambitious project. They worked on it for years with top talent. And they felt this is the minimum lovable product on which they will iterate. And if you would like to build product-ready solution similar to Math Notes, you can leverage existing stuff. MyScript.com have really good handwriting recognition system with bells and whistles. You can take a lot of shortcuts.